Light pens enable a position on a phosphorescent display of a Cathode Ray Tube (CRT) display to be identified. In a CRT display, an electron beam is moved across a screen in a raster scan, with the beam incident on a phosphorescent screen. Since the raster scan is performed at a known rate, the time between the start of the scan and the time at which a given position is excited by the beam is also known. Therefore, the time between the start of the scan and the time at which a light pen located at a position on the screen detects light may be used to determine the location of the light pen on the screen.
Light pens cannot be used with LCD displays due to variable latency in the digital interface between the computer and the display.
Alternatives to light pens, which may be employed with LCD screens, include system electromagnetic signal detection. For example, a grid of electromagnetic sensors embedded in the screen may be used to detect an electromagnetic field (such as an RF field) generated by a stylus, or may be used to sense variations in the magnetic field caused by the stylus. Alternatively, the electromagnetic field generated by pixel addressing may be detected by the stylus. Stylus systems are often required to provide position information not only in the X-Y plane of the screen, but also in the direction away from the screen (i.e. in ‘hover’ mode). Electromagnetic styluses typically perform poorly when the stylus is used in a ‘hover’ mode.
Alternatively, ultrasonic signals generated by a stylus may be detected by sensors around the periphery of the screen. Such schemes rely upon sophisticated software algorithms and require significant dedicated hardware, such as multiple sensors and data converters.
It would be useful to provide a simple, accurate and efficient optical stylus system for use with LCD screens.